This invention relates principally to a method of charging nickel-hydrogen and nickel-cadmium batteries, arid in particular to a method which prevents over-charging.
It is important to charge rechargeable batteries to their full charge conditions, and then to cease charging without over-charging. Over-charging has the detrimental effect of significantly reducing battery lifetime. In particular, nickel-hydrogen batteries are characteristically wear with respect to over-charging. It is also important to reliably charge batteries used under various conditions to full charge.
It should be noted that in this application, "cease charging" is not limited to the condition of zero charging current, but rather is taken in broader context to include conditions where full charge is detected and the charging current is reduced to a trickle current.
Two methods, (1) and (2), have been developed to detect a fully charged battery.
(1) .DELTA.V Method
This is a mettled of detecting full charge conditions by detecting a .DELTA.V drop in voltage from the peak voltage value. This method is described, for example, in Japanese Patent Disclosure 288740, 1986. This method utilizes the characteristic of fully charged rechargeable battery voltage to drop from a peak value. FIG. 1 shows voltage change over time during recharging of a rechargeable battery. Rechargeable battery voltage increases rapidly at first, voltage change subsequently decreases, voltage rises steeply near full charge voltage peaks at full charge, and voltage decreases from the peak value with further charging. Consequently, by detecting a .DELTA.V drop in voltage from the peak value, full charge can be detected.
On the other hand, the .DELTA.V method has the drawback that charging of over-discharged non-activated batteries is prematurely ceased by this method. This because, as shown in FIG. 2, nonactivated batteries have voltage characteristics with a voltage drop during the early period of charging. To avoid sensing a .DELTA.V drop at this early point, a method has been developed wherein charging is continued for a fixed time interval after beginning charging regardless of voltage change, then after the fixed time interval has elapsed, voltage drop from its peak value is sensed to cease charging. However, the method of detecting a .DELTA.V drop from the peak voltage to cease charging can significantly reduce battery lifetime for batteries, such as nickel-hydrogen batteries, that are weak with respect to over-charging. When these batteries are charged, the .DELTA.V method results in overcharging as shown by the hatched area of FIG. 1. For this reason, the .DELTA.V method has the drawback that batteries that are weak with respect to over-charging cannot be charged to an optimal condition.
(2) Peak Method
This method detects peak battery voltage to cease charging, and it is described, for example, in Japanese Utility-Model Publication No. 13253, 1985. This method has the feature that over-charging rechargeable batteries is reduced compared to the .DELTA.V method, (1). However, as shown in FIG. 3, when charging normally discharged batteries, charging can be completely stopped prior to reaching full charge. The reason for this, as shown by the broken line segments of FIG. 4, is that the curve representing battery voltage change at the early stages of charging approximates that near full charge. To prevent prematurely breaking off of the charge of normally discharged rechargeable batteries, this method also adopts the technique of charging continuously for a fixed time interval at the beginning of charge. For this reason, this method as well as method (1) have the drawback that fully charged batteries are over-charged.
A further problem with this method is that the length of time for continuous charging without sensing voltage at the beginning of charge must be longer than that for the .DELTA.V method. The reason for this, as shown In FIG. 3, is that the rising voltage curve for a normally discharged battery takes a particularly long time to level off. In method (1), the initial continuous charging time is set to prevent premature break off of charge for non-activated batteries. On the other hand, in method (2), It is necessary to continuously charge without sensing voltage until the voltage increase for normally discharged batteries levels off. Since the time for the voltage of normally discharged batteries to off is substantially long, it is necessary for the peak method to lengthen the initial continuous charge without sensing voltage. Consequently, this method can charge a normally discharged battery without over-charging, however, as shown in FIG. 5, this method has the drawback that a fully charged battery gets completely over-charged.
As described above, the .DELTA.V method (1) has the drawback that normally discharged rechargeable batteries are over-charged. Method (2) does not over-charge normally discharged rechargeable batteries but has the drawback that fully charged rechargeable batteries are substantially over-charged. Therefore, both methods (1) and (2) have the drawback that over-charging cannot be avoided for normally discharged rechargeable batteries and for fully charged rechargeable batteries.
The present invention was developed to solve the above mentioned problems. It is thus a primary object of the present invention to provide a battery charging method to optimally charge rechargeable batteries and to prevent over-charging and premature disconnection of rechargeable batteries used in various conditions.